MAZATLAN UNIVALVES 183 
times throughout. The apex in the adult is always flattened. 
The smallest specimen found, ‘03 in length, has much the 
aspect of a Navicella. The apex is terminal, spiral, of a very 
few whirls, involute, like atumid Planorbis, turned a little 
from the middle, away from the canal, and apparently sinistral. 
The head mark is at the opposite end of the shell. <A partition 
is soon placed across the spiral part, which then dropstoff at a 
variable period, leaving a horse-shoe cicatrix. In the speci- 
mens examined, the angle of adherence varied somewhat, as 
did the degree of revolution of the spire. The family is thus 
separated by another character from Patellide, in which the 
apex is never spiral, and from the true Calyptreide in which it 
is persistent. The first portion of the normal shell which is 
formed is smooth. Presently fine striz are developed ; after- 
wards, often quite suddenly, large ribs. There is nothing in 
the juvenile shell to shew whether it will develop into the 
flat or the conical form. The channel is not apparent in the 
very young shell; but soon becomes developed to an abnor- 
mally large size. The largest shell of the flattened form 
(including palmations) measures long. ‘96, lat. °89, alt. °18. 
A subconieal sp. 4 $3.) y Ls sath (ODmes a mmcelt 
The flattened form, which is rare, may be distinguished as 
S. L. var. panMatTa: #. planatd, costis magnis, rotundatis, ad 
marginem projicientibus, tntus cavatis, quasi palmatis.—It is 
probably the 8. denticulata of Whe. (non Quoy § Gaim.) Zeit. 
JF Mal. 1851);.0. 38, no. 136. 
"Hab.—Mazatlan, Philippi;—do. Melchers ; do. abundant ; 
L’pool Col. 
Tablet 833 contains the youngest specimen, Navicella-shaped, 
and an older one with the scar from the detached spiral part.— 
834, Ssp. jun. different ages, with spiral part visible.—835, 
9 sp. do. spiral part detached. 
Tablet 836 contains 3 sp. var. palmata, very few (11+) ribs.— 
837, 4 do. large and regular.—838, 5 do. irregular.—839, 4 do. 
ribs very numerous, (22,) smaller.—840, 4 do. interstices of 
palmations slightly crenate.—841, 5 do. interstices crenated.— 
' 842, 4 do. greenish tinge.—843, 5 do. light brown.—844, 4 do. 
very dark.—845, 3 do. nearly black. 
Intermediate form ; slightly palmated margin. Tablet 846 
contains 3 sp. approaching var. palmata.—847, 3 do. dark 
brown.—848, 3 do. white margin.—849, 3 do. light brown.— 
850, 2 do. chocolate colour. 
